Nonwoven fabrics are used in a wide variety of applications, where the engineered qualities of the fabrics can be advantageously employed. These types of fabrics differ from traditional woven or knitted fabrics in that the fibers or filaments of the fabric are integrated into a coherent web without traditional textile processes. Entanglement of the fibers or filaments of the fabric provide the fabric with the desired integrity, with the selected entanglement process permitting fabrics to be patterned to achieve desired aesthetics, and physical characteristics.
The term “hydroentanglement” generally refers to a process that was developed as a possible substitute for a conventional weaving process. In a hydroentanglement process, small, high intensity jets of water are impinged on a layer of loose fibers or filaments, with the fibers or filaments being supported on an unyielding perforated surface, such as a wire screen or perforated drum. The liquid jets cause the fibers, being relatively short and having loose ends, to become rearranged, with at least some portions of the fibers becoming tangled, wrapped, and/or knotted around each other. Depending on the nature of the support surface being used (e.g., the size, shape and pattern of openings), a variety of fabric arrangements and appearances can be produced, such as a fabric resembling a woven cloth or a lace.
The term “spunbonding” refers to a process in which a thermoplastic polymer is provided in a raw or pellet form and is melted and extruded or “spun” through a large number of small orifices to produce a bundle of continuous or essentially endless filaments. These filaments are cooled and drawn or attenuated and are deposited as a loose web onto a moving conveyor. The filaments are then partially bonded, typically by passing the web between a pair of heated rolls, with at least one of the rolls having a raised pattern to provide a bonding pattern in the fabric. Of the various processes employed to produce nonwovens, spunbonding is the most efficient, since the final fabric is made directly from the raw material on a single production line. For nonwovens made of fibers, for example, the fibers must be first produced, cut, and formed into bales. The bales of fibers are then processed and the fibers are formed into uniform webs, usually by carding, and are then bonded to make a fabric.
Hydroentangled nonwoven fabrics enjoy considerable commercial success primarily because of the variety of fiber compositions, basis weights, and surface textures and finishes which can be produced. Since the fibers in the fabric are held together by knotting or mechanical friction, however, rather than by fiber-to-fiber fusion or chemical adhesion, such fabrics offer relatively low tensile strength and poor elongation. In order to overcome these problems, proposals have been advanced to entangle the fibers into an already existing separate, more stable substrate, such as a preformed cloth or array of filaments, where the fibers tend to wrap around the substrate and bridge openings in the separate substrate. Such processes obviously involve the addition of a secondary fabric to the product, thereby increasing the associated effort and cost.
Another method for improving strength properties is to impregnate the fabric with adhesive, usually by dipping the fabric into an adhesive bath with subsequent drying of the fabric. In addition to adding cost and effort to the process, however, addition of an adhesive may undesirably affect other properties of the final product. For instance, treatment with an adhesive may affect the affinity of the web for a dye, or may otherwise cause a decline in aesthetic properties such as hand and drape as a result of increased stiffness.
Because of the above discussed problems associated with hydroentangled webs, the hydroentangling practice as known by those skilled in the art heretofore has been principally limited only to staple fibers, to prebonded webs, or to filaments of only an extremely small diameter. The hydroentanglement of webs of filaments that are continuous, of relatively large diameter, or higher denier has heretofore not been considered feasible. Conventional wisdom suggests that long, large diameter, continuous filaments would dissipate energy supplied by entangling water jets, and thereby resist entanglement. An additional factor suggesting that continuous filaments could not be sufficiently hydroentangled to form a stable, cohesive fabric is that as the filaments are continuous they do not have loose free ends required for wrapping and knotting. Yet another problem in the hydroentangling process as presently known and practiced in the industry is associated with production speed limitations. Presently known methods and apparatuses for hydroentangling filaments are not able to achieve rates of production equal to those of spunbonding filament production.
Various prior art patents disclose techniques for manufacturing nonwoven fabrics by hydroentanglement. U.S. Pat. No. 3,485,706, to Evans, hereby incorporated by reference, discloses methods and apparatus for formation of nonwoven fabrics by hydroentanglement. This patent describes the fiber physics involved in the production of such fabrics, noting that entangled fibers within the fabrics are restrained from movement by interaction with themselves and with other fibers in the fabrics. Such interaction is stated as being caused by the manner in which the fibers are interengaged so as to cause them to interlock with one another. This patent is principally directed toward the entanglement of fibers, but reference is made to entanglement of continuous filament webs. It is believed that the tested samples comprised loose filament webs, and were subjected to laboratory scale treatments that did not appropriately model continuous processing of filamentary webs. It is additionally noted that this patent does not distinguish between fiber entangling physics of the staple or textile length fiber examples set forth therein, and that of the continuous filament examples. It is believed that when subjected to the testing described in the patent, the fabric samples did not provide results that would define differences in their construction. Use of cut hand sheets of spunbond webs is believed to have rendered the filaments thereof in a discontinuous form. Additionally, fiber ends of the cut edges were not constrained, as would be the case during hydroentanglement of an intact continuous filament web. As a consequence, it is believed that the continuous filaments referred to in this patent were actually more in the nature of long staple fibers, and as such, responded to the energy of water jets as staple fibers, that is, recoiling and wrapping around one another. U.S. Pat. No. 3,560,326, to Bunting, Jr., et al., is believed to be similarly limited in its teachings, and thus it is not believed that this patent meaningfully distinguishes between the fiber entangling physics of relatively short fibers (i.e., staple or textile length), and continuous filament examples set forth therein.
U.S. Pat. No. 4,818,594, to Rhodia, contemplates hydroentanglement of fibers having diameters on the order of 0.1 to 6 microns, which by virtue of their micron-sized diameters are clearly formed by melt-blowing, as opposed to spunbonding.
U.S. Pat. No. 5,023,130, to Simpson et al., discloses the use of plexifilamentary fibrous webs which are known in the art as being instantaneously bonded during production. This patent is limited to the use of a very fine mesh forming screen, and the use of water jet pressures that are in excess of 2,000 psi in the initial forming stations.
U.S. Pat. No. 5,369,858, to Gilmore et al., discloses a nonwoven fabric comprising at least one layer of textile fibers or net polymeric filaments, and at least one web of melt-blown microfibers, bonded together by hydroentangling. This patent specifically contemplates that a spunbonded fabric is employed as a substrate for entangling of secondary melt-blown or carded webs, with the patent further contemplating formation of apertures of two differing sizes in the fabric.
As is recognized in the art, the use of particular types of polymeric fibers or filaments can be desirable depending upon the desired physical characteristics of the nonwoven fabric formed from the fibers or filaments. In particular, polyethylene filament webs are desirable for application such as facings, coverstock, and similar applications because of the softness and drapeability the polyethylene provides. A drawback associated with the use of polyethylene filament webs for such applications is the low tensile strength the filaments exhibit. Polypropylene or polyester filament webs are typically strong in comparison to polyethylene, but products formed from polypropylene or polyester filament are relatively stiff in comparison to polyethylene filament products.
It can be difficult to combine polyethylene webs with other stronger webs to produce a product that is both soft and strong. Bonding temperature differences ordinarily make it difficult or impossible to thermally bond a web that might be produced in a continuous process that includes, for example, two filament beams, one producing polyethylene and the other producing polypropylene. A temperature selected to bond the polyethylene is insufficient to bond the polypropylene portion. While it is possible to thermally bond the layers using two thermal bonding steps, thermally bonding the polypropylene as a first step undesirably stiffens the polypropylene. The polyethylene layer added to such a web thus exhibits undesirable stiffness. The resultant laminated product would consist of the polyethylene layer and a relatively stiff reinforcing layer.
As noted above, various methods for making nonwoven fabrics are well-known. In general, these fabrics are made from bonded fibers or filaments, or combinations thereof. In spunbonding, a thermal plastic polymer is melt-extruded into a plurality of continuous filaments and deposited on a conveyor. The filaments are then continuously thermally point-bonded to one another using calender rolls. As also noted, formation of nonwoven fabrics by hydroentanglement entails the use of high intensity, fine jets of water which are impinged on a web, causing the fibers to entangle and form a coherent mechanically bonded structure.
In spunbonding, it is known that the tensile strength of the fabric of a given basis weight can be increased by decreasing the size of the filament. In addition, the uniformity of a fabric of a given basis weight also generally increases with reduced filament size. However, reduced filament causes a reduction of production output and efficiency, whether or not the web is formed as a single layer, or in multiple layers.
In hydroentanglement, the fiber web that is initially deposited consists of individual unbonded fibers, and the web therefore tends to be fragile. For this reason, the pressure of the initial water jets impacting the web must be kept low to avoid excessive fiber displacement, with subsequent jets operating at higher pressures used to more significantly entangle the fibers. This requirement of “pre-entangling” the web with low initial pressure jets decreases the efficiency of the entangling process. One known method proposed for resolving this problem is to support the upper exposed surface of the unbonded web with a perforated screen during entanglement, but disadvantageously involves the use of additional equipment.
In addition, conventional hydroentanglement fabrics as they presently exist are not considered durable, in the sense that they are not launderable. Also, conventional fabrics cannot be subjected to modern jet dyeing processes which involve high flow rates of the treating liquid. These limitations limit the commercial applications of such fabrics and thereby significantly affect their economic value. Proposals have been advanced to treat the finished fabric with a curable binder. This, however, increases the processing effort and cost of the product. Further, the binder may have an adverse effect on the final fabric properties, such as softness and drapeability, as well as the ability to dye the fabric.
Heretofore, durable, launderable nonwoven fabrics have traditionally relied upon relatively high levels of thermal bonding, surface treatments to bond the surface of the fabrics, or stitch bonding techniques to provide a stabilizing network for tying down fiber ends. U.S. Pat. No. 5,192,600 and No. 5,623,888 disclose stitch bonding technology for the production of nonwoven fabrics, with the bulky fabrics described therein stated as being useful in a variety of apparel and industrial end uses. U.S. Pat. No. 5,288,348 and No. 5,470,640 disclose high loft, durable nonwoven fabrics which are produced by serial bonding of layers, followed by an all-over surface bonding with a greater bond area than any of the intermittent bonding steps.
U.S. Pat. No. 5,587,225 describes the use of hydroentangling to bind an interior layer of cellulosic short fibers to outer layers of crimped continuous filaments. While the end product is described as “knit-like” and durable, the product is intended to survive only one laundry cycle, losing up to 5% of the original basis weight during the first washing. While the spunbond outer layers are described as being prebonded, the use of crimped continuous filaments is specifically contemplated, with reliance on the crimped configuration to assist in the retention of short, cellulosic fibers in the entangled matrix. It will be appreciated that the crimping process requires either a mechanical step, or the use of bi-component fibers which develop latent crimp as an aspect of processing, and thus the use of standard spunbond fabrics is not contemplated. Additionally, this patent contemplates the use of a short staple fiber inner layer to increase the opacity and visual uniformity of the final product.
The present invention further contemplates a process for formation of a laminated nonwoven fabric, comprising polymeric filament layers exhibiting differing properties. There is, therefore, an as yet unresolved need in the industry for a process of hydroentangling continuous filaments of relatively large denier, that is, filaments having diameters greater than those generally achieved by melt-blowing formation. Also, there is a heretofore unresolved need in the industry for a hydroentangled nonwoven fabric comprised of continuous filaments of relatively large denier. Further, there is an unresolved need in the industry for an apparatus for producing a nonwoven web comprised of hydroentangled continuous filaments of relatively large denier, and for a method and apparatus for hydroentanglement capable of rates of production substantially equal to spunbonding production rates. A further aspect of the present invention contemplates production of highly durable, dyeable nonwoven fabric made of hydroentangled continuous filaments. The process employs spunbonded webs that are fully stabilized by thermal point bonding with high pressure jets utilized to separate the filaments from the thermal bond points, freeing the filaments for entangling by water jets. Notably, the process contemplates use of multiple prebonded spunbond layers to form a composite web of substantial basis weight, up to 600 g/m2 (grams per square meter).